In a cellular communication system, when there is service transmission between two User Equipments (UEs), for example, service data from a user equipment 1 (UE1) to a user equipment 2 (UE2) firstly are transmitted to a Base Station (or referred to as a Node B or an evolved Node B) of a cell in which the UE1 is located via an air interface, the base station transmits the user data to a base station of a cell in which the UE2 is located via a core network, and the base station then transmits the above service data to the UE2 via an air interface. The similar processing flow is adopted in the service data transmission from the UE2 to the UE1. As shown in FIG. 1 (a), when the UE1 and the UE2 are located in the same cellular cell, though the two UEs are covered by the cell of the same base station, the data are still required to be transferred via the core network when transmitted, and one time of the data transmission still consumes two wireless spectrum resources.
Thus it can be seen that, if the user equipment 1 and the user equipment 2 are located in the same cell and in a close distance, the above cellular communication method is apparently not optimal. Moreover, in fact, with the diversification of the mobile communication services, such as the popularization of the applications like social networks and electronic payment and so on in the wireless communication system, the demand for service transmission between short-distance users is ever increasing. Therefore, the Device-to-Device (D2D) communication mode has increasingly attracted widespread attention. As shown in FIG. 1 (b), D2D refers to that the service data are directly transmitted by a source user equipment to a target user equipment via an air interface without forwarding of the base station and the core network, and it also can be called Proximity Service (ProSe). For the users in the near field communication, the D2D not only saves the wireless spectrum resources but also reduces the data transmission pressure of the core network.
In the cellular communication, when two UEs perform communication, as a general rule, the UE itself does not know the location of the opposite-side UE, and a connection of the two UEs is established through a network side device (such as the base station or the core network device). But, for the D2D communication, a premise for establishing the communication link is the mutual discovery between the UEs. However, in the related art, with respect to the problem of how the UE discovers an adjacent UE (or also can be called companion discovery or device discovery), no effective solution has been put forward at present.